I think Dave answered pretty well but I'll shed more light on it.
I know 403 errors when the server's file permissions are set to NOT allow public access to the file(s) in question.
Assuming your code and their API directions are correct (I didn't check) :
This is not something you as the client can correct. It requires server access to change permission settings which allow you the client access.
If your API code is not correct, then correct it
You may want to get an HTTPsocket to upload a file to your own server using php first to make sure your boundaries are correct, etc.
There might be a thread on this in the networking section.
I would offer more help but I'm totally swamped; Not only am I behind my own production schedule but I got suckered into making a website / online business for a family member for free
Ps. I really want to WE to be working reliably !
Doing this stuff in php, CSS, HTML, etc. is boring and takes me a long time!!!!
Gotta look more at WE soon and put it to the test.